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When the LTC4300 first receives power on its V
either during power-up or during hot swapping, it starts
in an undervoltage lockout (UVLO) state, ignoring any
activity on the SDA and SCL pins until V
rises above
2.5V. For the LTC4300-2, the part also waits for V
rise above 2V. This ensures that the part does not try to
function until it has enough voltage to do so.
During this time, the 1V precharge circuitry is also ac
tive and
forces 1V through 100k nominal resistors to the
SDA and SCL pins. Because the I/O card is being plugged
into a live backplane, the voltage on the backplane SDA
and SCL busses may be anywhere between 0V and V
Precharging the SCL and SDA pins to 1V minimizes the
worst-case voltage differential these pins will see at the
moment of connection, therefore minimizing the amount
of disturbance caused by the I/O card.
Once the LTC4300 comes out of UVLO, it assumes that
SDAIN and SCLIN have been hot swapped into a live sys
tem and
that SDAOUT and SCLOUT are being powered up
at the same time as
itself. Therefore, it looks for either a
STOP bit or bus idle condition on the backplane side to
indicate the completion of a data transaction. When either
one occurs, the part also verifies that both the SDAOUT
and SCLOUT voltages are high. When all of these condi
tions are met, the input-to-output connection circuitry is
, joining the SDA and SCL busses on the I/O card
with those on the backplane.
Connection Circuitry
Once the connection circuitry is activated, the functionality
of the SDAIN and SDAOUT pins is identical. A LOW forced
on either pin at any time results in both pin voltages being
LOW. SDAIN and SDAOUT enter a logic HIGH state only
when all devices on both SDAIN and SDAOUT force a HIGH.
The same is true for SCLIN and SCLOUT. This important
feature ensures that clock stretching, clock arbitration
and the acknowledge protocol always work, regardless
of how the devices in the system are tied to the LTC4300.
Another key feature of the connection circuitry is that it
provides bidirectional buffering, keeping the backplane
and card capacitances isolated. Because of this isolation,
the waveforms on the backplane busses look slightly
different than
the corresponding
card bus waveforms, as
described here.
Input to Output Offset Voltage
When a logic LOW voltage, V
, is driven on any of
the LTC4300’s data or clock pins, the LTC4300 regulates
the voltage on the other side of the chip (call it V
to a slightly higher voltage, as directed by the following
= V
+ 50mV + (V
/R) • 100
where R is the bus pull-up resistance in ohms. For
example, if a device is forcing SDAOUT to 10mV and if
= 3.3V and the pull-up resistor R on SDAIN is 10k,
then the voltage on SDAIN = 10mV + 50mV + (3.3/10000)
100 = 93mV. See the Typical Performance Characteristics
section for curves showing the offset voltage as a function
of V
and R.
Propagation Delays
During a rising edge, the rise time on each side is deter
mined by the combined pull-up current of the LTC4300
boost current and the bus resistor and the equivalent
capacitance on the line. If the pull-up currents are the
same, a difference in rise time occurs which is directly
proportional to the difference in capacitance between the
two sides. This effect is displayed in Figure
1 for V
3.3V and a 10k pull-up resistor on each side (50pF on
one side and 150pF on the other). Since the output side
has less capacitance than the input, it rises faster and the
effective t
is negative.
There is a finite propagation delay, t
, through the con-
nection cir
cuitry for falling waveforms. Figure 2 shows
the falling edge waveforms for the same V
, pull-up
resistors and equivalent capacitance conditions as used
in Figure 1. An external NMOS device pulls down the
voltage on the side with 150pF capacitance; the LTC4300
pulls down the voltage on the opposite side, with a delay
of 55ns. This delay is always positive and is a function of
supply voltage, temperature and the pull-up resistors and
equivalent bus capacitances on both sides of the bus. The
Typical Performance Characteristics section shows t
as a function of temperature and voltage for 10k pull-up